fallback.js

const staticAssetsFromFilesystem = { /* Express static asset serving middleware */ }; 
const fallbackToStaticAssetsFromS3 = { /* Custom middleware that calls out to S3 */ }; 
app.use(`/public`, staticAssetsFromFilesystem, fallbackToStaticAssetsFromS3);

headers.js

'X-Served-From': 'fallback'

logging.js

appLogger.log("Fallback static asset retrieved from: " + url);